A cone with a base of radius r and height H is cut by a plane parallel to and h units above the base. Find the volume of the solid (frustrum of a cone) below the plane.
I assume since you posted this in the calculus section, you're looking for a solution involving a volume of revolution.
consider the line $\displaystyle y = \frac{r}{H} x$ rotated about the x-axis from $\displaystyle x = h$ to $\displaystyle x = H$
$\displaystyle V = \pi \int_h^H \left(\frac{r}{H} x\right)^2 \, dx
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